1. Field of the Invention
This invention relates to the feeding of flexible sheets, and particularly to the feeding of pieces of fabric, such as lace, to a sewing position.
2. Description of Related Art
Garments, such as underclothes, have previously been manufactured by passing suitably-shaped pieces of fabric ("cut parts") to a machinist, who then overlays and/or folds them as required, and passes them manually through a sewing machine. The machinist forms seams, binds the edges of the cut parts and adds lace and elasticated waistbands, where necessary. The accuracy of the positioning of the seams and location of the lace, etc. relies on the machinist's skill.
Systems have been proposed for automating at least a part of the manufacturing process by using a robot to move a cut part to a work station, such as a sewing machine. The movement of the cut part is effected by a "gripper" which is mounted on the output shaft of the robot, and which makes contact with the upper surface of the cut part.
The movement of the cut part may be monitored by an electronic vision system, which controls the robot accordingly.
A machinist sewing scalloped lace on to a garment will guide the lace and the underlying fabric through the sewing machine, monitoring their positions by eye, so that the needle stays at a reasonably constant distance from the edge of the lace. The sewing direction must always be parallel to the tangent to the curved edge at each point.
If that procedure were to be attempted using an electronic vision system, a very complicated system would be required because the system must continuously determine what orientation of the lace and the fabric is needed, at every instant, to maintain the sewing line parallel to the edge. Furthermore, very little space is available around the sewing foot of the sewing machine for accommodating vision system components (i.e. light sources and photodiodes). Also, the operation of determining where the edge of the lace lies is very difficult for an electronic vision system to effect, bearing in mind that the lace will be lying on top of a cut part of a material which will very probably be the same color as the lace.
One possible method of sewing on such lace automatically would be to store data defining required movements of the lace and the fabric based on the assumption that scalloped lace has an absolutely regular, cyclically-varying outline. Unfortunately, that is far from the case. Lace is dimensionally very unstable, and a sewing method which relies on absolutely cyclical movements could very rapidly get completely out of phase with the actual shape of the lace. The stitching would then veer between being too far across the lace and being completely off its edge.